For working tools run by internal combustion engines generally two-stroke engines are used, mainly due to their low weight and simple design. Also, the crankcase scavenging enables a lubrication system independent of position, in which the engine is lubricated by oil which is added to the air-fuel mixture scavenged through the crankcase. The all-position lubrication system is necessary e.g. for chain saws since they are to be used in a lot of different working positions. Two-stroke engines for mopeds and motor-cycles generally have a so called tuned exhaust system. Reflecting pressure pulses from the exhaust system will press scavenging gases back into the cylinder so that the engine's scavenging losses are reduced. In total this means that both the power output and fuel consumption can be improved in comparison with a non-tuned exhaust system. However, in order to function the tuning requires very large lengthes of pipe in the exhaust duct. Such a muffler for a chain saw would be at least half a meter long and consist of a first conically expanding duct section by approximately 8 degrees, and a second conically narrowing section by approximately 12 degrees. Thereafter an absorption muffler should be connected in order to reach reasonable sound-levels. As mentioned above such a muffler is built on reflecting pressure pulses as well as a low total fall of pressure. Regarding working tools it has turned out that such a muffler will be far too large and heavy. This even if the pipe system is provided with a lot of curves. For, a working tool must be very light-weight, compact and handy in order to serve its purpose. Consequently, tuned exhaust systems are normally not used for working tools. Instead they have light-weight and compact mufflers in which the sound mainly is damped by throttling in the muffler. A larger cylinder volume is used to reach the preferable effect. Owing to the fact that there are great differences between the lay out of two-stroke engines with tuned exhaust systems and without tuned exhaust systems, it is difficult to transfer experiences from one area to another.
A well-known problem with two-stroke engines is their relatively high fuel consumption caused by high scavenging losses, i.e. scavenging gases which go straight out into the exhaust system. This also results in high emissions, especially from hyrdocarbons. As mentioned above, the difficulties to overcome this problem are especially big for two-stroke engines with light-weight and compact mufflers, i.e. with non-tuned exhaust systems. The high extent of emissions from hydrocarbons also results in certain problems when using a muffler with catalytic conversion. For, the very high energy of the exhaust gases leads to a very high heat generation in the catalytic converter as well as in the surrounding muffler. This high extent of heat generation could mean that the conversion ratio in the catalytic converter must be kept down. Consequently, the high scavenging losses would increase the fuel consumption at the same time as they could complicate a cooperation with an exhaust catalytic converter.